WL#4151: Remove __NT__ from server code

Affects: Server-7.x   —   Status: Complete   —   Priority: Medium

MySQL depends on NT features.  It can no longer run on pre-NT servers.

Therefore, the __NT__ mess can go away, which will simplify code and reduce errors.

See also:  BUG#31814, with a user-supplied patch which has a good idea of
defining the required windows version via the _WIN32_WINNT macro.